The zona pellucida contains a set of glycoproteins that are thought to mediate the species-specificity of sperm binding to oocytes. We have developed methods to isolate the zona pellucida as an intact structure and have determined its protein content. Immature mouse follicles are prepared and grown in culture for seven days. During this maturation period, the zona pellucida is formed around oocytes. The proteins of the oocyte and zona are isolated and displayed on SDS gels and two-dimensional O'Farrell gels. Using a biological assay we can demonstrate that the dissolved zona will inhibit species-specific sperm binding. We are now in the process of characterizing the physico-chemical properties of zona protein and determining which of them mediate the specific sperm binding.